When I sew of Olivia, everything is a bit big if I use the same size as her clothes. When I sew for my self I have just the oposite problem. In ready made skirts I wear an 8, when I made a skirt I use a size 14. Big difference!

Get her grandma to measure her for the dress as you can enlarge size 4. Measure around neck, upper arms, chest, waist, hips and length of where her dress should be.

I agree that you should get measurements. height and weight are a good start as new patterns tend to have them on them. My DGD is almost 4, and the size 3 childs pattern is still too big around the neck.
Sometimes patterns have an ease added in and that makes them run bigger than ready made. But start with measurements. She my be smaller at the shoulders for one size but need the next size in length.

I would definately get her measurements...at least the chest and waist. You say that she is tall...maybe that is why she is wearing a 5...the 4's are too short.

As carpenters always say - measure twice - cut once. I also believe that patterns run big. "Wearing ease" is what they call it and you can end up looking like you are wearing a sack. Adding length to a pattern is fairly easy - and it does sound like she is tall - though we don't know how tall gramma is. Best bet -besides getting a new size pattern and knowing she will grow into it eventually - is to make an inexpensive prototype and seeing how it works. 5/8" seams are forgiving and adjustable!

When you are talking child sizes it's still cute. When I go by my measurements and realize that I need a size 26 pattern (or thereabouts) it gets really disheartening. I learned in class to take measurements on the body you are sewing for and also measure the same parts on the pattern itself. That will tell you how much ease you will have or for multi-size patterns, which size to actually cut.

My daughter was one of those kids who was "deceptive". She wore a size 5 right as she turned 3 years old (she's 6'2" now). She was tall but skinny. Size 5 for length in store bought clothing, size 2 or 3 in width. Definitely get measurements and measure the pattern pieces before you cut. Not all patterns, designers, etc. were created equal. Some had more ease than others. Tailoring ahead of time saves a lot of grief later.